Mucus’ velocity

Mucus gel is propelled toward the epiglottis by a two-phase ciliary beat cycle. Forward mucus movement occurs during the effective or power phase of the cycle, when cilia fully extend and traverse an arc perpendicular to the epithelial surface (Fig. 5.24). Claw-like structures, 25-35 nm long, project from each cilia tip and appear to assist in the mechanical transfer of momentum from cilia to mucus gel. Maximum mucus velocity depends on the extent cilia penetrate the epiphase during the power phase, periciliary and mucus gel viscosity, and cilia density. 

Mussatto, D.J., Garrad, C.S. and Lourenco, R.V (1988). The effect of inhaled histamine on human tracheal mucus velocity and bronchial mucociliary clearance. Am. Rev. Respir. Dis. 138 775-779. 

Mucociliary Clearance Mucociliary clearance operates by the coordinated movements of cilia, which sweep mucus out of the lungs towards the pharynx where it is swallowed. There is an inverse relationship between mucus velocity and airway generation, which relates to the lower percentage of ciliated cells, shorter cilia, lower ciliary beat frequency and lower number of secretory cells in the peripheral airways [121]. The reported tracheal mucociliary clearance... 

Measurements of linear mucus velocities in the human trachea were conducted mainly with two methods inhalation or instillation. 

Nonciliated cells separate fields of ciliated epithelial cells from each other. Synchronized ciliary movement, with a beat frequency in human proximal airways under normal conditions of 8-15 EIz, propels mucus along the mucociliary escalator at a rate of up to 25 mm/min. Beat frequencies appear to slow to roughly 7 Hz in more distal airways. Cilia move in the same direction and in phase within each field but cilia in adjacent fields move in slightly different directions and are phase shifted. These beat patterns result in metachronal waves that steadily move mucus at higher velocities ( -12-18 mm/min) than would be achievable by summing the motion of individual cilia. 

MaePherson, L. M. D., and Dawes, C. (1991). Urea concentration in minor mucus gland secretions and the effect of salivary film velocity on urea metabolism by streptococcus vestibularis in an artificial plaque./Periodont. Res. 26, 395- 01. 

Lichtiger, M., Landa, J.F. and Hirsch, J.A. (1975). Velocity of tracheal mucus in anesthetized women undergoing gynaecologic surgery. Anesthesiology 42 753-756. 

There have been a number of studies of the thickness and velocity of the mucus layer, with different results. Dalham reported a thickness of 5 Min in the trachea of rats. Similarly, Alder et al. reported 10 Mm and less in cats, and Comroe reported 10-15 urn. Velocity has been measured at 13.5 mm/min in rats, 0-35 mm/min in cows and 5-14 mm/min in dogs, 10.5 3.7 mm/min in cats, and 15 mm/min in the human trachea and 3.75 mm/min in the human main bronchus. ... 

Clearance in the upper, or ciliated, region is governed by the rate of mucus transport along the airways. These rates have been measured in the human nose and in dogs, rats, and other species. Asmundsson and Kilbum, Hilding, and Iravani established that mucociliary clearance rates increase from the distal bronchi toward the trachea. Because bronchial openings retard mucus flow, bifurcations receive an accumulation of mucus and associated particles. The rate of mucus production and mucus thickness and velocity vary from one person to another. Thickness increases and velocity decreases greatly when some toxic elements are present in the airway. 

As noted earlier, air-velocity profiles during inhalation and exhalation are approximately uniform and partially developed or fully developed, depending on the airway generation, tidal volume, and respiration rate. Similarly, the concentration profiles of the pollutant in the airway lumen may be approximated by uniform partially developed or fully developed concentration profiles in rigid cylindrical tubes. In each airway, the simultaneous action of convection, axial diffusion, and radial diffusion determines a differential mass-balance equation. The gas-concentration profiles are obtained from this equation with appropriate boundary conditions. The flux or transfer rate of the gas to the mucus boundary and axially down the airway can be calculated from these concentration gradients. In a simpler approach, fixed velocity and concentration profiles are assumed, and separate mass balances can be written directly for convection, axial diffusion, and radial diffusion. The latter technique was applied by McJilton et al. 

Figure 1 Estimated concentrations of liposomally encapsulated peptide (CM3) in the airway surface liquid (ASL) immediately after completion of nebulization for various simulated subject ages, mucus production rates, and tracheal mucous velocities. Generation 0 corresponds to the trachea, while the terminal bronchioles are generation 14. (From Ref. 1, with permission.)...

The mucociliary escalator functions when inhaled particles between 2 to 10 pm are deposited on the sticky mucous lining of the tracheobronchial tree and are propelled upward by the movement of this mucous layer in response to the beat of the cilia on the ciliated epithelial cells. This phenomenon is made possible because the mucous layer is biphasic, consisting of a watery solution in direct contact with the epithelia cells in which the cilia are free to beat. This watery solution is covered by a stickier, more adhesive gel layer that can trap and hold inhaled particles. The cilia beat in the wall layer at a rate between 1000 and 1500 strokes per minute such that at the point of their maximum upward velocity, the tips of the cilia come in contact with the gel layer to propel it upward. All recovery strokes subsequently occur in the water layer. This mechanism moves the mucus upward at a rate between 1 and 3 cm/minute. Ultimately, the mucus reaches the pharynx, where it is swallowed. Disease states that either alter the mucus-producing properties of cells in the tracheobronchial tree or decrease ciliary activity will obviously have a deleterious effect on this important clearance mechanism. 

This approach involves the measurement of the distribution of swimming distances made by the spermatozoa in various chambers, such as curved microchannels in microchips [87] or cervical mucus-fiUed capillaries [3, 82, 106]. The swimming distances are a measure of the progressive velocity and the percent motile cells in the sperm population. 

The relation between the viscosity and elasticity of the secretions is one of the determining factors in transport velocity. If the gel phase is in practice the only one really transported, the sol phase creates a low-resistance milieu where the cilia can beat, an environment that is essential for transport in the direction of the upper airways. One of the most important rheological properties of mucus is viscosity. Viscosity is resistance to flow and represents the capacity of a material to absorb energy while it moves. Elasticity is the capacity to store the energy used to move or deform material. The ratio between viscosity and elasticity appears to be an important determinant of the transport rate (6,10). Mucus transport by ciliary beating is influenced by the viscoelastic and surface properties of the mucus. Theoretical models suggest that a decrease in the ratio of viscosity to elasticity can result in an increase in mucociliary transport (13). 

The small transport surface in the main airways (which could lead to the accumulation of secretions) is compensated in normal subjects by greater velocity related to higher ciliary beat frequency. The most important transport mechanism of mucus in the bronchial tree is mucociliary transport. This movement takes place by coordinated activity of cilia that cover the bronchial surface of the airways. Ciliated cells are found in the airways from the trachea to the terminal bronchioles. Each cell contains about 200 cilia, all of which end in little claws. The cilia beat in the direction of the oropharynx with a frequency of about 8 to 15 Hz. The claws of the cilia reach the mucus gel layer and push this layer towards the oropharynx. The recovery beat in the direction of the bronchioles takes place only in the periciliary sol layer. The cilia normally move the mucus at 1 mm/min in smaller airways and at up to 2 cm/min in the trachea (1,4,6). 

Longer cilia should be able to clear mucus faster because they can generate a greater forward velocity. In smaller airways the cilia are generally shorter and fewer in number than in the large bronchi, and even though the cilia beat frequency may be comparable, the rate of momentum transfer to the mucus is proportionately less. 

Fig.4 shows the maximum shear strength versus the mucus film thickness. The average mucus film thickness was decided as foUows. The mucus of 10 pi was set on a base glass of 5cm sq., and pressed and spread by another glass plate. The average film thickness was decided from the spread area and the volume of 10 pi. The pulling velocity was 1.26 mm/s,... 

The mean linear velocity of mucus, obtained from ten studies using instillation, was 14 5,5 mm/min, whereas that obtained from 13 studies using inhalation was 5.3 1.3 mm/min (2). The velocities measured after bronchoscopic instillation were higher and more variable than those measured after inhalation. This could be due to stimulation of the mucociliary transport system, either by... 

Foster et al. (32) also measured mucociliary clearance in the main bronchi after inhalation of radiolabeled particles, using a gamma-camera. For healthy nonsmokers, a linear velocity of 2.4 0.5 mm/min was measured in the bronchi, compared with 5.5 0.4 mm/min in the trachea. Furthermore, they (32) found a correlation between tracheal and bronchial mucous velocities. There is no direct information available about mucous velocities in more distal airways. From the foregoing observation, it can be assumed that linear mucous velocity will decrease in more peripheral airways. By making assumptions about the properties of mucus during its transport from distal airways towards the trachea, mucous velocity has been modeled by several authors (33-36). From these calculations the mucus-transport velocity in terminal bronchioli could be more than three orders of magnitude lower than in the U achea. 

Foster WM, Langenback EG, Bergofsky EH. Lung mucociliary function in man interdependence of bronchial and tracheal mucus transport velocities with lung clearance in bronchial asthma and healthy subjects. Ann Occup Hyg (Inhaled Particles V) 1982 26 227-244. 

The transport velocity of mucus-simulant gels is directly related to mucus s elasticity and the depth of the periciliary fluid, and it is inversely related to mucus viscosity (1). An ideal viscoelastic ratio may exist for optimal mucociliaiy interaction an increase in viscosity or a decrease in elasticity would result in a reduced transport rate. Transport by cough or airflow interaction depends inversely on viscosity, elasticity (spinnability), and adhesivity (1). Mucus that is elastic, rather than viscous is transported well by ciliary action, but less well by coughing (2). 

The direction of flow of mucns is from the small airways to the larynx. To prevent drovming in mucus, the ciliary beat frequency and mncns s velocity increase from distal to proximal airways (112). At the pharynx, the mncns is either... 

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